Application of perturbation theory in large configuration interaction calculations

Rayleigh-Schrödinger perturbation theory has been applied through fifth order in the energy, to the problem of estimating the roots of the secular equation in large configuration interaction calculations. The NO₂⁺, O₃ and H₂O molecules are used as test cases, with accuracy as good as 0.01 eV, with appropriate choice of zero order problem.     

Variational calculation of vibrational energies of triatomic molecules using SCF optimized modes

A self-consistent field optimization of the vibrational coordinates for nonlinear triatomic molecules is presented. The optimal coordinates are obtained by making a three-dimensional rotational transformation of the normal modes and determining the rotation angles as those for which the SCF energy is stationary. The utility of the optimized coordinates in full variational calculations of vibrational energies is studied for the molecules of H₂O, O₃, H₂D⁺, H₂T⁺, and D₂T⁺. For H₂O and O₃, the optimization procedure leads to the local mode representation. It is shown that the use of the optimal coordinates in variational calculations allows a large reduction of the dimension of the Hamiltonian matrix to be diagonalized in order to reach convergence.     

Single-determinantal open- and closed-shell FSGO calculations for some diatomic molecules

Open-shell single-determinantal calculations are reported here for the molecular species H₂, Li₂⁺, N₂, O₂ (triplet), O₂^2?;, O₂^?, O₂²⁺, O₂⁺, and F₂; corresponding closed-shell calculations are reported for the species H₂, N₂, O₂ (singlet), O₂^2?, O₂²⁺, and F₂. The floating spherical Gaussian orbital (FSGO ) method was employed. The calculated trend in bond lengths of isonucleic diatomic molecules is in agreement with experiment. For heteronucleic diatomic molecules, however, the experimental trend in bond lengths is not obtained; in this connection, the effect of lone pairs on bond length is discussed. The dissociation energies of H₂ and Li₂⁺ are evaluated. The energy gap between the triplet and singlet states of the oxygen molecule is calculated to be 8.96 eV compared to the experimental value of 4.54 eV.     

Ab initio calculations of activation energy of the reaction of hydrogen exchange on strongly acidic centers

Ab initio calculations of fragments of the potential energy surfaces of hydrogen exchange reactions between H₂, CH₄, and alanine molecules and the H₃O⁺ ion were performed by the restricted Hartree-Fock method, at the second-order Møller-Plesset level of perturbation theory, and by the method of coupled clusters using the 6–31G^* and aug-cc-pVDZ basis sets. The one-center synchronous mechanism of hydrogen exchange reaction was studied and the activation energies and structures of transition states were determined. It was found that the geometric parameters of the H₂ and CH₄ molecules in the transition states are close to those of the H₃ ⁺ and CH₅ ⁺ ions. The higher the proton affinity of the reacting molecule in the reaction studied the lower the activiation energy of hydrogen exchange. The one-center mechanism studied can be used to describe the high-temperature solid-state catalytic isotope exchange (HSCIE) reaction. The results ofab initio calculations of synchronous hydrogen exchange between the H₃O⁺ ion and hydrogen atoms in different positions of the alanine molecule are in good agreement with experimental data on the regioselectivity and stereoselectivity of the HSCIE reaction with spillover-tritium.     

Molecular calculations with the MODPOT,VRDDO, and MODPOT/VRDDO procedures. IV. Boron hydrides and carboranes

Reference completely ab initio 6–3G and nonempirical 3G/MODPOT (ab initio effective core model potential) LCAO -MO -SCF calculations (using the same valence atomic orbital basis) were performed for a series of boron hydrides (B₄H₁₀, B₅H₉, B₅H₁₁, and B₆H₁₀) and a test 3G/MODPOT + VRDDO (variable retention of diatomic differential overlap for charge conserving integral prescreening) calculation were also performed for B₅H₉, B₆H₁₀, and B₁₀H₁₄. The agreement between the ab initio 6–3G and the corresponding 3G/MODPOT calculations was excellent for valence orbital energies, gross atomic populations, and dipole moments. The results also compared favorably to previous ab initio minimum STO basis results of Lipscomb and coworkers. The 3G/MODPOT + VRDDO calculations verified that for such spatially compact molecules (such as boron hydrides, which are fragments of polyhedra), the VRDDO procedure does not result in a noticeable savings in computer time for molecules of the size and shape of B₅H₉ and B₆H₁₀, in contrast to the savings previously realized for organic molecules of comparable atomic size. However, the agreement in calculational results between the 3G/MODPOT and the 3G/MODPOT +VRDDO results was still as extremely close as it had been for the organic molecules. 3G/MODPOT calculations were also carried out for B₈H₁₂, B₉H₁₅, B₁₀H₁₄, B₁₀H₁₄^?2, 1,2-C₂B₄H₆, and 1,6-C₂B₄H₆ and the results compared to the previous minimum STO basis results. For B₁₀H₁₄, the 3G/MODPOT +VRDDO method led to savings in computer time of 28% over the 3G/MODPOT method itself. The agreement of the 3G/MODPOT results with available experimental photoelectron spectral data for B₅H₉ and 1,6-C₂B₄H₆ was as good as that of the previous ab initio minimum STO basis calculations.     

Core-valence correlation effects for molecules containing first-row atoms. Accurate results using effective core polarization potentials

The accuracy of employing effective core polarization potentials (CPPs) to account for the effects of core-valence correlation on the spectroscopic constants and dissociation energies of the molecules B₂, C₂, N₂, O₂, F₂, CO, CN, CH, HF, and C₂H₂ has been investigated by comparison to accurate all-electron benchmark calculations. The results obtained from the calculations employing CPPs were surprisingly accurate in every case studied, reducing the errors in the calculated valence D _e values from a maximum of nearly 2.5 kcal/mol to just 0.3 kcal/mol. The effects of enlarging the basis set and using higher-order valence electron correlation treatments were found to have only a small influence on the core-valence correlation effect predicted by the CPPs. Thus, to accurately recover the effects of intershell correlation, effective core polarization potentials such as the ones used in the present work provide an attractive alternative to carrying out computationally demanding calculations where the core electrons are explicitly included in the correlation treatment. Received: 11 May 1998 / Accepted: 27 July 1998 / Published online: 28 October 1998     

LCAO Xα calculation of the ionization energies of small molecules

LCAO Xα calculations of the ionization energies of the molecules C₂H₄, H₂O₂, CO₂ and HF are reported. The agreement with photoelectron data is shown to be excellent and in several cases superior to that obtained from ab initio Hartree-l'ock calculations using the ΔE(SCF) procedure.     

2h-1p Cl calculations of ionization potentials of small molecules: Corrections to the koopmans theorem

2h-1p Cl calculations of ionization potentials have been performed on N₂, F₂, HNO, HOF, trans-N₂H₂, cis-N₂H₂, H₂NN, O₃, F₂O and C₂N₂, both with ground state and relaxed orbitals. Several inversions present at the KT level are recovered and a good qualitative agreement with experimental values and results of accurate calculations is achieved. Interaction with the 2h-1p configurations is found to be responsible for the major correlation effects.     

An experimental and computational approach to pH‐dependent self‐aggregation of poly(2‐isopropyl‐2‐oxazoline)

Besides temperature, self‐aggregation of poly(2‐isopropyl‐2‐oxazoline) (PIPOX) can also be triggered via pH in aqueous solution (25 °C, pH > 5). Lowest energy structures and interaction energies of PIPOX with H₃O⁺, OH^?, and H₂O were calculated by DFT methods showed that, in addition to their ability to protonate PIPOX, H₃O⁺ ions had strong interaction with both water and PIPOX in acidic conditions. H₃O⁺ ions acted as compatibilizer between PIPOX and water and increased the solubility of PIPOX. OH^? ions were found to have stronger interaction with water compared to PIPOX resulting in desorption of water molecules from PIPOX phase and decreased solubility, leading to enhanced hydrophobic interactions among isopropyl groups of PIPOX and formation of aggregates at high pH. Results concerning the effect of end‐groups on aggregate size were in good agreement with statistical mechanics calculations. Moreover, the effect of polymer concentration on the aggregate size was examined. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 210–221     

Interstitial and lattice substituted models of an expanded ice-I hlattice

An ice-I _hlattice (O?O=0.286 nm) with small molecules placed interstitially and with some placed at lattice positions was investigated by the CNDO/2-MO technique. The interstitial molecules included H₂, N₂, O₂, HF, CO₂, H₂O, NH₃, CH₄ and CH₂O, whereas those involved in lattice substitution included HF, NH₃ and H₂O. From the calculations it is found that all interstitial and lattice substituted systems are stabler than the sum of the components, the enhanced stability depending on the system. Generally, lattice substituted systems are stabler than the corresponding interstitial models. Charges residing on the atomic positions reflect the amount of interaction with the matrix as well as indicating how the change in charge would facilitate other interactions with the solvent.     

Experimental and Theoretical Study of Hydrogen Atom Abstraction from C2H6 and C4H10 by Zirconium Oxide Clusters Anions

The reactions of anionic zirconium oxide clusters Zr_xO_y^- with C₂H₆ and C₄H₁₀ are investi-gated by a time of flight mass spectrometer coupled with a laser vaporization cluster source.Hydrogen containing products Zr₂O₅H^- and Zr₃O₇H^- are observed after the reaction. Den-sity functional theory calculations indicate that the hydrogen abstraction is favorable in the reaction of Zr₂O₅^- with C₂H₆, which supports that the observed Zr₂O₅H^- and Zr₃O₇H^- are due to hydrogen atom abstraction from the alkane molecules. This work shows a newpossible pathway in the reaction of zirconium oxide cluster anions with alkane molecules.     

Multipole expansion of diatomic overlap

The systematic extension of Ruedenberg's expansion formula proposed in Part I [1] is applied to a series of diatomic and polyatomic molecules (BH, NH, HF, Be₂, C₂, F₂, CO, BH₃, CH₄, NH₃, H₂O, HCN and H₂CO). In general, good agreement with the results of full SCF calculations with the same minimum STO basis set is achieved. Thus, the errors due to this integral approximation scheme called MEDO (Multipole Expansion of Diatomic Overlap) are almost negligible compared to those introduced by basis set truncation.     

Total cross sections for electron scattering by polyatomic molecules at 10–1000 eV: C2H2, C2H4, C2H6, C3H6, C3H8 and C4H8

A model complex optical potential (composed of static, exchange, polarization and absorption terms) is employed to calculate the total (elastic and inelastic) electron-atom scattering cross sections from the corresponding atomic wave function at the Hartree-Fock level. The total cross sections (TCS) for electron scattering by their corresponding molecules (C₂H₂, C₂H₄, C₂H₆, C₃H₆, C₃H₈ and C₄H₈) are firstly obtained by the use of the additivity rule over an incident energy range of 10–1000 eV. The qualitative molecular results are compared with experimental data and other calculations wherever available, good agreement is obtained in intermediate-and high-energy region.     

Silicon dioxide surface reconstruction stimulated by adsorption interaction

The MM2 molecular mechanics method is used for calculations on the adsorption complex formed on the surface of silicon dioxide due to adsorption of gases such as H₂O, O₂, NH₃, HF. The results of modeling are in good agreement with experimental data; these results explain why film thickness decreases after adsorption and support our hypothesis about reconstruction of the surface layer stimulated by the adsorbed molecule.     

Qualitative and quantitative analysis of intermolecular interactions in xanthenedione derivatives

The existence of intermolecular interactions and the conformational geometry adopted by molecules are related to biological activity. Xanthenedione molecules are promising and emerging antioxidants and acetylcholinesterase inhibitors. To examine the role of different functional groups involved in the intermolecular interactions and conformational geometries adopted in xanthenediones, a series of three substituted xanthenediones have been crystallized [9‐(3‐hydroxyphenyl)‐3,3,6,6‐tetramethyl‐3,4,5,6,7,9‐hexahydro‐1H‐xanthene‐1,8(2H)‐dione, C₂₃H₂₆O₄, 9‐(5‐bromo‐2‐methoxyphenyl)‐3,3,6,6‐tetramethyl‐3,4,6,7‐tetrahydro‐2H‐xanthene‐1,8(5H,9H)‐dione, C₂₄H₂₇BrO₄, and 3,3,6,6‐tetramethyl‐9‐(pyridin‐2‐yl)‐3,4,6,7‐tetrahydro‐2H‐xanthene‐1,8(5H,9H)‐dione, C₂₂H₂₅NO₃] and their intermolecular interactions analyzed via Hirshfeld analysis. The results show that all the derivatives adopt the same structural conformation, where the central ring has a shallow boat conformation and the outer rings have a twisted boat conformation. The intermolecular interactions in the molecules are predominantly O—H…O, C—H…O and π–π interactions. The optimized structures of the derivatives from theoretical B3LYP/6‐311G** calculations show a good correlation with the experimental structures. The lattice energy involved in the intermolecular interactions has been explored using PIXELC.     

NH4+ Deprotonation at Interfaces Induced Reversible H3O+/NH4+ Co-insertion/Extraction

Ion insertions always involve electrode-electrolyte interface process, desolvation for instance, which determines the electrochemical kinetics. However, it′s still a challenge to achieve fast ion insertion and investigate ion transformation at interface. Herein, the interface deprotonation of NH₄⁺ and the introduced dissociation of H₂O molecules to provide sufficient H₃O⁺ to insert into materials′ structure for fast energy storages are revealed. Lewis acidic ion-NH₄⁺ can, on one hand provide H₃O⁺ itself via deprotonation, and on the other hand hydrolyze with H₂O molecules to produce H₃O⁺. In situ attenuated total reflection-Fourier transform infrared ray method probed the interface accumulation and deprotonation of NH₄⁺, and density functional theory calculations manifested that NH₄⁺ tend to thermodynamically adsorb on the surface of monoclinic VO₂, and deprotonate to provide H₃O⁺. In addition, the inserted NH₄⁺ has a positive effect for stabilizing the VO₂(B) structure. Therefore, high specific capacity (>300 mAh g⁻¹) and fast ionic insertion/extraction (<20 s) can be realized in VO₂(B) anode. This interface derivation proposes a new path for designing proton ion insertion/extraction in mild electrolyte.     

Evaluation of AM1-calculated radical cation ion-neutral complexes

AM1 semiempirical molecular orbital calculations are reported for 20 ion-neutral complexes, including hydrogen-bonded complexes, presumably involved in the gas-phase unimolecular decomposition of simple organic radical cations. The systems investigated are [C₂H₄O₂]^˙+, [C₂H₅NO]^˙+, [C₂H₆O]^˙+, [C₂H₆O₂]^˙+, [C₃H₆O]^˙+, [C₃H₆O₂]^˙+, [C₃H₈O]^˙+, and [C₃H₈O₂]^˙+. The AM1 results are compared with ab initio molecular orbital calculations at different levels of theory up to MP3/6-31G(d, p)//SCF/6-31G(d) + ZPVE and the available experimental data. AM1 fails to predict some local minima and the equilibrium geometries calculated for several complexes are found to be qualitatively different from those predicted by the ab initio calculations. However, reasonable agreement is generally found for the stabilization energies of the complexes toward dissociation into their loosely bound components. © John Wiley & Sons, Inc.     

A First-Principles Study on the Adsorption of Small Molecules on Arsenene: Comparison of Oxidation Kinetics in Arsenene,Antimonene, Phosphorene,and InSe

Arsenene, a new group-V two-dimensional (2D) semiconducting material beyond phosphorene and antimonene, has recently gained an increasing attention owning to its various interesting properties which can be altered or intentionally functionalized by chemical reactions with various molecules. This work provides a systematic study on the interactions of arsenene with the small molecules, including H₂, NH₃, O₂, H₂O, NO, and NO₂. It is predicted that O₂, H₂O, NO, and NO₂ are strong acceptors, while NH₃ serves as a donor. Importantly, it is shown a negligible charge transfer between H₂ and arsenene which is ten times lower than that between H₂ and phosphorene and about thousand times lower than that between H₂ and InSe and antimonene. The calculated energy barrier for O₂ splitting on arsenene is found to be as low as 0.67 eV. Thus, pristine arsenene may easily oxidize in ambient conditions as other group V 2D materials. On the other hand, the acceptor role of H₂O on arsenene, similarly to the cases of antimonene and InSe, may help to prevent the proton transfer between H₂O and O− species by forming acids, which suppresses further structural degradation of arsenene. The structural decomposition of the 2D layers upon interaction with the environment may be avoided due to the acceptor role of H₂O molecules as the study predicts from the comparison of common group V 2D materials. However, the protection for arsenene is still required due to its strong interaction with other small environmental molecules. The present work renders the possible ways to protect arsenene from structure degradation and to modulate its electronic properties, which is useful for the material synthesis, storage and applications.